I have a 12 by 12 poured patio tha butts up to the house. I plan on placing the deck over the patio and extending it out over the yard on two of the three sides. I will not be attaching to the house. The deck height will be approximatly 18 inches. Is there anything special that needs to be done for the placks sitting in the yard. I have no idea if the patio is frost proff. THe final deck size will be around 20 by 20.

Frost protected slabs are rare, 95% of them are not. Most slabs are 4" thick. If it is any thicker than that, it may be frost protected. I attached a link for building over an existing slab, there is a great example in that link.

www.deckplans.com/FAQ/05_FAQ_Patio.html

Here's another example

www.deckplans.com/gallerydetail.asp?deck...;amp;TypeSearch2=Yes